Molecular mechanisms that lead to the progressive loss of muscle size and strength with age have not been elucidated. Elderly individuals appear particularly susceptible to muscle loss following injury and associated processes such as surgery. Muscle damage is repaired directly and through recruitment of satellite cells, myoblasts that proliferate and differentiate in response to muscle degeneration thereby regenerating damaged muscle fibers. Muscle regeneration is impaired in aged individuals. This research will establish if age-related changes in the ability of satellite cells to proliferate and differentiate in response to muscle damage contribute to loss of muscle with age. As the insulin-like growth factors (IGFs) are the only trophic factors identified to date that stimulate proliferation and myogenic differentiation, the role of the IGFs in controlling satellite cell function during aging will be determined. IGF gene expression is induced in activated satellite cells during muscle regeneration in vivo and during myoblast differentiation in vitro, and IGFs appear to act in an autocrine and/or paracrine manner on satellite cells to promote differentiation. Moreover, activation of IGF gene expression in regenerating muscle may be required not only for satellite cell differentiation, but also for effective reinnervation of newly formed muscle fibers. Our preliminary data suggest that satellite cells derived from aged human donors proliferate more slowly and do not differentiate to the same extent as those derived from young donors, and that IGF gene expression is significantly reduced in the aged myoblasts. We propose to quantitate IGF gene expression during in vitro differentiation of primary human myoblasts derived from different aged donors. As the transcription factor myogenin appears to be required for IGF stimulation of differentiation, myogenin gene expression will also be quantitated. The effect of exogenous IGF treatment on aged myoblasts will then be determined by monitoring growth rate, myoblast fusion, and expression of markers of differentiation such as myogenin and myosin heavy chain. These experiments will determine if inherent changes in IGF gene expression occur in activated satellite cells with age and whether these changes are correlated with changes in satellite cell function. We will also analyze IGF and myogenin expression in vivo in response to muscle injury in rats of different ages, induced by nerve transection. Muscle regeneration will be monitored by analyzing the reexpression of fetal isoforms of myosin heavy chain, and the effect of localized, exogenous IGF treatment will be assessed. Finally, signal transduction pathways responsive to IGFs with a potential role in regulating satellite cell function will be examined using a nondifferentiating muscle cell line. Changes in protein phosphorylation, as well as in the activity of regulators of myoblast- specific gene expression under different growth conditions will be identified. Taken together, this proposal will ascertain the role of locally produced IGFs in muscle regeneration following injury and examine whether changes in their production by satellite cells contribute to the loss of muscle mass with age. The feasibility of using IGFs as therapeutic agents to maintain muscle mass in the elderly can then be evaluated.